If the neutral theory is true, then molecular clocks are fine. If the molecular... there's a lot of controversy about whether they are true, and I mean, by and large, my feeling about clocks is that the neutral theory is near enough true to make the clocks fairly reliable. But there has been a big problem with bacteria. To discover whether the clock in bacteria goes at the same rate as a clock in mammals, let's say, because we don't have any fossil bacteria. But the problem has ultimately been solved by a group of people, the only one I actually know is Nancy Moran, who was a student in Ann Arbor when I was there, who have got the clock calibrated by realising that aphids - greenfly and things like that - have endosymbiotic bacteria, and you can show... and they're transmitted in the egg by the aphid, so that the mother gives it to her daughters. And it turns out that the phylogenetic tree of the aphids and of the bacteria are identical. And since you can date the branching of the aphids, by amber and things of that kind, you know the branching of the bacteria, and you can ask how fast is the clock going. And it really is going surprisingly similarly to that in the aphid, I mean, it really is a very striking observation, to me a bit unexpected, for various reasons. But it really does look as if the bacterial clock is not greatly different in rate to the... the clock for the rest of the world.

[Q] Shouldn't you be able to use the molecular clock to test the idea that there was a Cambrian explosion, by looking at say, I don't know, annelid and mollusc DNA and seeing whether you really can trace them back to the Cambrian and whether they would trace back to some time in the pre-Cambrian?

I think the clock obeys Maynard Smith's rule about numbers in biology, which is that they're always right to a factor of two. And when I say I believe the molecular clock, I do believe it to a factor of two, I do not believe it to a factor of 10%. And to get Cambrian versus pre-Cambrian, which is, after all, 600 million years ago, I think the error is too big to enable you to do that. It is clear, from the molecular clock, that... that the radiation of the main groups was reasonably rapid, but it could have been a 100 million years. I mean, it could have been slow by genetic standards but fast by paleontological ones.